High silicon cast iron



April 14, 1964 Filed May 9, 1965 Area of Excess Brihleness W. A. LUCEETAL HIGH SILICON CAST IRON Area of Poor Corrosion Resistance PoorCorrosion M Resistance Walter A. Luce Glenn W. Jackson I Earl Ryder Mars6. Fonlana INVENTORS United States Patent Ofiice 3,129,095 Patented Apr.14, 1964 3,12%,095 EHGH SELECQN CAST IRON Walter A. Luce, Glenn W.Jackson, and Earl Ryder, Dayton, and Mars G. Fontana, Columbus, Ohio,assignors to The Duriron Company, lino, Dayton, {)hio, a corporation ofNew Yorl:

Filed May 9, 1963, Ser. No. 279,140 11 Claims. (Ql. 7-l26) Thisinvention pertains to high silicon cast iron alloys possessingextraordinary resistance to chemical attack throughout a wide spectrumof corrosive environments; and it pertains especially to alloys whichexhibit substantially improved chloride resistance while maintaininggood resistance in other media. The novel alloys are particularly usefulin areas where relatively high operating temperatures and criticalconcentrations of the corrosive media are encountered. concomitantlywith such corrosion resistance, the novel alloys possess adequatethermal shock resistance, mechanical strength and hardness such thatthey oifer important practical advantages over previously availablealloys in the fabrication of chemical processing equipment and in otherfields where corrosion problems are encountered, especially wherecombined with erosion problems and variable temperature conditions.

It is well recognized that hydrochloric acid and particularly thechloride ion provide some of the most highly corrosive serviceconditions that a metal or alloy may be called upon to withstand. Whilechloride ion resistance is not the only consideration, such resistancein alloys of the type under discussion here is a basic factor,necessarily to be considered. The generally good chloride corrosionresistance of prior high silicon iron alloys under moderately severeconditions of concentration and temperature is well-known. Typicalalloys of this type which have been in use for some time are those soldunder the trade names Duriron and Durichlor by the Duriron Company.Duriron has a nominal composition of about 14.5% silicon, 0.65%manganese, 0.85% carbon, the balance being iron. Durichlor has about thesame composition except for the substitution of about 3% molybdenum fora corresponding amount of iron. These alloys are economical to use andprovide unexcelled resistance under many conditions to most organic andmineral acids and other corrosives. While the effectiveness of 'Durironabove normal temperatures drops ofi appreciably in hydrochloric acidservice, Durichlor remains elfective in practice for most concentrationsof this acid up to temperatures of 50 to 60 C. and up to 80% C. whereimpurities, such as ferric or other oxidizing ions, are absent. However,where highly oxidizing metal ions are present, such as ferric, cupric,or mercuric, even Durichlor corrodes at a faster rate than is generallyconsidered commercially acceptable. At temperatures above 60 C., thechoice of practical materials is largely restricted to super-alloys, thelatter containing relatively small amounts of iron and being predominantin nickel, chromium and molybdenum. These are expensive and usuallyhighly strategic materials. Chlorimet-Z, for example, which is the tradename for a typical commercially available alloy of this type, containsabout 62% nickel, 32% molybdenum, 3% iron and a maximum of about 1%silicon. Such an alloy has excellent resistance to severe corrosives ofa reducing nature, e.g. boiling hydrochloric acid in all concentrations.But even this superalloy is attacked quite readily under aeratedconditions or where any substantial amount of ferric chloride or otheroxidizing agent is present. This specific difiiculty can be corrected byusing a different type of alloy, for example Chlorimet-3 which dilfersfrom Chlorimet-2 principally in that chromium is substituted for aboutone half of the molybdenum content of the latter alloy. This change,however, substantially reduces the resistance of the alloy tohydrochloric acid as compared with Chlorimet-Z under ordinary,non-oxidizing conditions. Thus although better than Chlorimet-Z in thepresence of oxidizing contaminants, the loss in corrosion resistance isonly partially compensated by Chlorimet-3.

There are a number of other metals and alloys, including such puremetals as titanium and zirconium, which have good corrosion resistanceunder severe service, but these are generally subject to disadvantagesof excessively high cost, problems of availability, or the further factthat while they are useful in certain limited or specific service, theydo not provide over-all effective resistance in corrosive media of bothhighly oxidizing and highly reducing types. Furthermore resistance toabrasion and erosion of these is limited.

Thus there has long been a need in industry for less expensive alloyswhich will stand up under a wide variety of the more severely corrosiveas well as erosive environments to which industrial equipment isnecessarily exposed, or tor alloys which will permit operation underconditions not practical heretofore but nevertheless desirable toemploy. One of the objects of the present invention accordingly is toprovide an economical alloy having improved corrosion resistance undersevere service conditions. While effective resistance to severe chlorideconditions, such as prolonged exposure to strong hydrochloric acid, isparticularly important, it is a further object of the invention toprovide alloys which at the same time have superior corrosion resistancein many other corrosive media frequently encountered in industry. Alongwith these objectives, the invention further provides alloys which areeasier to found and from which sound castings of good strength can beobtained. A further object is to provide alloys having excellent surfacecharacteristics for highly erosive and abrasive application.

The new alloys are high silicon cast irons modified by the inclusion ofcertain elements, more particularly chromium, most of which are held insolid solution at least in the preferred embodiments. As used herein,the term high silicon cast iron designates iron-silicon alloys in whichthe silicon content is above the level, generally about 6% to 8%silicon, where a definite break or transition occurs in the matrix as aresult of ordering of the silicon and iron atoms, tending to producebrittleness in the alloy, and carbon for the most part is present in theform of free graphite randomly dispersed in the essentially continuoussilico-ferrite matrix. In all cases, chromium in prescribed amount isincluded in the alloy, as this provides the key to the startlingimprovement aiforded by the invention. Other additions optionallyincluded for specific applications are molybdenum, and sometimes boronand columbium. The alloys may contain without appreciable adverse effectthe usual manganese and trace impurities of sulfur, phosphorus and otherelements generally occurring in conventional high silicon irons.Elements such as aluminum, cobalt, antimony, tin, manganese, silver,lead, titanium totalling in some cases as much as about in the alloys,have little or no adverse effect upon the corrosion resistance in amajority of corrosive environments. The same is true, but to a markedlylesser extent, of additions of nickel or copper. However, in all casesthe corrosion rates in the more severe corrosive environments areincreased appreciably by the inclusion of more than small or residualamounts of nickel or copper.

The unique combination of remarkably improved corrosion resistance andadequate mechanical properties which characterize the alloys of thisinvention is achieved by the discovery that chromium, which ordinarilydecreases corrosion resistance in reducing type media, has just theopposite effect in high silicon cast iron alloys of the type discussedabove, provided the chromium is present within critical limits. Priorhigh silicon cast iron founders have deliberately avoided the inclusionof chromium in significant amount, with the result that if it waspresent in prior alloys, residual amounts only, i.e. less than 1% atmost, were involved. There is good reason for this, since, as mentionedabove, previous work in this field teaches that corrodibility,particularly in hydrochloric acid and similar reducing media, increaseswith chromium content, and the exclusion of this element has accordinglybeen the rule for corrosion service applications.

The addition of chromium alone to high silicon cast iron, in the amountshereinafter specified, shows superior results for example as a materialfor impressed current anodes in the cathodic protection of steelstructures or equipment. This is true in salt water as well as hightemperature fresh water service, both of which have presented extremelytroublesome metal corrosion problems in the past. Heretofore, standardhigh silicon cast irons such as Duriron and Durichlor mentioned abovehave proved to be among the most practical types of alloys available forthis protective anode service, but these alloys are limited toconditions where the temperature does not exceed around 60 C. in thecase of Durichlor or 50 C. in the case of Duriron. The use of the newchromiumcontaining high silicon cast iron alloys is not so limited, aswill appear more fully hereinafter.

Those alloys within the scope of the invention which exhibit stillgreater resistance to chemical corrosion under more severe conditionsinclude, in addition to chromium, supplemental amounts of molybdenum.The addition of molybdenum produces a very substantial increase inresistance, particularly in hydrochloric acid, but must be carefullycontrolled to avoid adverse effect on the mechanical properties of theresulting alloys. The further addition of boron when molybdenum ispresent is also sometimes desirable to facilitate casting of the metalin the foundry and to reduce any tendency of the alloy to spall.

The novel alloys, by virtue of their high proportion of iron and siliconare notably economical to use in comparison with the super-alloys orpure metals heretofore required for comparably severe serviceconditions. Moreover the new alloys are readily handled by thosefamiliar with high silicon cast iron founding and present no newproblems in casting practices.

While corrosion resistance is a major consideration, it is obvious thatthe alloys must also possess adequate mechanical properties forpractical utility, and frequently the dictates of maximum corrosionresistance are in direct conflict with those of useful mechanicalproperties. The alloys of this invention are unique in respect to combining excellent chemical resistance over a broad spectrum of corrosiveconditions with practical mechanical strength and thermal shockresistance. These mechanical properties are dependent in large measureon the presence of free graphite in the alloy matrix, and it is anessential '4 characteristic of the invention alloys that they are truecast irons, as contrasted to steels. The carbon level and the nature ofthe carbon content in the invention alloys is accordingly important.

Like other high silicon ferrous alloys, the cast irons of the inventionare characterized by rather extreme hardness, to the extent that theyare not readily machinable in most instances. The novel alloys, however,are quite fluid in their molten state and thus readily permit casting ofintricate parts with very small cross sections, as well as the moreuniform, heavier shapes. The castings may be ground to provide afinished surface and any tendency to spall is largely eliminated, wherethis is a problem, by the inclusion of boron. Also the alloys are easilypulverized and classified according to mesh size for application to abase metal, using powdered metal spray techniques, to form an adherent,hard, dense, corrosion resistant cladding or facing material. Thosealloys within the invention having maximum corrosion resistance andincluding molybdenum have a Rockwell C. hardness rating of up to around58 and provide excellent bearing surfaces. The straightiron-silicon-chromium alloys, without molybdenum, show hardness readingsof around 45 Rockwell C. Minimum tensile strength of at least 12,000p.s.i. is obtained in all of these compositions, which value isconsidered to be a permissible low limit, and generally higher tensilestrengths obtain in the new alloys, particularly in the preferredcompositions. It is however very diflicult to make suitable tensiletests without premature failure in a brittle alloy system like the highsilicon cast irons because of misalignment in tensile test apparatus orthe influence of slight imperfections in the test specimen. A transverseload value for a given test specimen is thus considered to be a moresuitable method in evaluating these relatively brittle alloys. As thereis no ASTM or other standard governing the procedure for a suitabletransverse load test, the following has been established by theinventors to provide a suitable basis for comparison. A test specimen iscast in the form of a one inch square bar, thirteen inches long. This isplaced on twelve inch centers in a testing machine, loaded at the centeruntil breakage occurs and the load recorded in pounds. A roughrelationship exists here between transverse load and tensile strength,such that the tensile strength of a specimen can be approximated bymultiplying the transverse load value by fifteen.

The corrosive media in which the new alloys are useful include not onlythe reducing type corrosives, such as hydrochloric acid and chloride ionenvironments, but oxidizing type corrosives as well. This includes moreparticularly nitric and sulfuric acids, as well as phosphoric, aceticand chromic acids, and the respective anions of these agents. The alloysare highly resistant to the common caustic reagents also, and showdefinite improvement in this respect over previous high silicon ironalloys. The alloys are useful not only under moderately corrosiveconditions of temperature and concentration in these various agents, butunder conditions which have previously made the use of knowniron-silicon alloys uneconomical.

In general, the compositional analyses of the preferred alloys of theinvention fall within the following component ranges:

Many of the advantages of the preferred alloys can be obtained in usefuldegree within the following broader ranges of the respective componentswhich, however, are found to be critical to the practical attainment ofthe unique combination of chemical and mechanical properties of theinvention alloys:

It will be understood from What has already been stated and from thediscussion to follow that adjustment of individual component percentageswithin the indicated limits in both the over-all and preferred rangesjust mentioned will be necessary to obtain maximum benefits for specificapplications or uses of the alloys. However, it should be borne in mindthat in all cases it is important to provide a cast iron, i.e. one inwhich free graphite is dispersed throughout the matrix, since thisstructure is found essential in the alloys here disclosed to obtainingthe desired mechanical properties.

In order to provide a basis for the better understanding of thiscorrelation or adjustment of component percentages, reference will bemade to the accompanying drawing illustrating graphically, in a plot ofsilicon percent vs. chromium percent, the limits to be observed inaccordance with the teaching of the invention.

The area represented by the quadrilateral abc-d in the drawingdelineates the limits of chromium and silicon in the compositions of thenovel :alloys. The area thus bounded constitutes the boundary of a phasediagram which is divided by a line b'c' into two regions. This line isdefined by the formula:

Percent Si0.7(percent Cr) 7.4=0

and represents the compositional level of chromium and silicon at whichthe graphitizing tendency of silicon abruptly stops and carbides canreadily form. Carbon in the alloys above the solid solution limit ofabout 0.1 to 0.2% (depending on the alloying elements present) willexist as free graphite in the region to the left of line b'c' and thusof cource produce a cast iron. Substitution of the chromium and siliconvalues in the above expression will thus give a positive value of Xunder this condition. This line remains constant irrespective of thetotal carbon present above the solid solution limit. In the preferredalloys a total combined carbon in solid solution and as free graphite ofabout 1.0% is normal, however, since the upper carbon limit isinfluenced by the actual silicon content as well as the presence ofcertain other elements, the upper limit may reach about 1.5% beforerejection of the carbon begins to occur during solidification. In theregion to the right of line b'-c' carbides are readily formed, as thecarbide forming tendency of chromium overcomes the graphitizing tendencyof silicon. In order to produce a cast iron in this particular region,sufficient carbon must first be added to satisfy the carbide-formingtendency and then additional carbon is necessary to provide the freegraphite.

Thus line b-c' delineates two regions on a phase diagram where to theleft of the line only a homogeneous silico-chromium-ferrite matrix andgraphite are in equilibrium while to the right, the matrix material,chromium carbides and graphite (provided the carbon level is adequate)are in equilibrium. When considering the region .to the right of linebc', therefore, it is apparent that the ability for graphite to beformed depends on the amount of silicon, chromium and carbon present. Atcarbon contents below about 1% it is impossible for graphite to formunder any condition in this region, but it can be made to occur byraising the carbon level to some predetermined point depending on thesilicon and chromium levels. It has been established by the inventorsthat where the total amount of carbon exceeds the value determined bythe formula:

C=1.660.063 (percent Cr) 0.089(percent Si) 6. then free graphite willexist and a cast iron structure obtained.

The foregoing considerations also apply generally where molybdenum ispresent up to about 1%, but above that level molybdenum becomes additivewith the chromium in respect to the amount of carbon which is tied up,and for such alloys the sum of chromium and molybdenum contents must betaken into consideration.

For purposes of reference, all of the alloy compositions referred toherein are tabulated in Table I and the alloys themselves areaccordingly referred to only by their letter or number designationthereafter in subsequent tables.

TABLE I Per- Per- Per- Per- Per- Per- Per- Per- Alloy cent cent centcent Alloy cent cent cent cent Si Cr G M0 Si Cr 0 M0 The presence orabsence of free graphite in various al loys at different silicon andchromium levels is illustrated in Table 11.

TABLE II Graphite Determination at Various Levels of Silicon, Chromiumand Carbon Alloy: Remarks F Free graphite. T Do.

Y Do. S No graphite. BB Free graphite. GG Do.

Z Do. AA Some free graphite. DD Do.

EE D0.

FF Do. V Do. CC No visible free graphite. W No graphite. X Do.

G Do. I Do.

M n Do. L Do. HH Some graphite. NN Free graphite.

Free graphite exists in alloysat the high silicon 17%) level, nominal'1%carbon, for chromium levels up to about 12% to 13%, due to the highgraphitizing effect of the silicon. Compare alloys F, Z, EE and X. Asthe silicon level is decreased, so must the chromium content also bedecreased, and at minimum silicon (12%), with more than 6% to 7%chromium free graphite disappears. Compare alloys Y, V and W. At thepreferred level of silicon (14% to 15%), nominal carbon of 1%, graphiteis prevent up to about 10% to 11% chromium (see alloy DD), but virtuallyno free graphite is found over 11% chromium (see alloy CC). However, astotal carbon is increased, graphite again appears at higher chromiumlevels. Thus at 13.25% silicon, 11.35% chromium, nominal 2% carbon, thegraphite structure is obtained, as indicated for alloy HH. Even at lowsilicon (11.83%), high chromium (15.11%), free graphite is found in thematrix where the total carbon is nominally 3% (see alloy NN).

Other factors also enter into consideration in determining the scope ofthe alloy compositions to obtain the unique properties of the inventionalloys, and further illustrate the criticality of the compositionallimits. As shown in Table III, low transverse load values become adetermining factor at the higher silicon and chromium levels,notwithstanding additional carbon. This brittleness begins to besignificant at about 15% silicon, and above 17% there is insufficientmechanical strength and thermal shock resistance in the alloys to makethem practical. In the case of chromium, increased brittleness appearsat around and becomes determinative at chromium levels. (See Table III.)

TABLE III Mechanical Properties Practical considerations in industrydictate that even for anode service, which is one of the least demandingfrom the standpoint of mechanical properties, the alloy material fromwhich the anodes are cast should provide a minimum transverse load valueof around 800 pounds and deflection of 0.025". Short of this, excessivebreakage of the anodes occurs simply from handling during storage andshipment. Thus, whereas a preferred composition within the scope of theinvention, such as alloy T in Table III, shows a transverse strength of1291 p.s.i. and deflection of 0.036", an alloy differing from this onlyin that it contains about 3% more silicon and thus falls just outsidethe indicated limits of the invention, has less than half the strengthand deflection of the preferred alloy. (See alloy F.) Quite apart fromcorrosion resistance, therefore, alloys such as alloy F are impractical.

At the lower limits of the given composition ranges, poor corrosionresistance in the alloys becomes chiefly determining. For silicon, theminimum practical level is around 12% and for chromium it is about 2%.Preferably silicon is at least 14% and chromium at least 3%. The resultsof corrosion tests on various alloys, both within and outside of thescope of the invention for comparison purposes are given in Table IV.The abbreviations used in this and other tables hereinafter relating tocorrosion rates have the following meanings:

40% BN--40% boiling nitric acid 10% BN-l0% boiling nitric acid 30% BS30%boiling sulfuric acid 30% S 80-30% sulfuric acid at 80 C.

% BH-20% boiling hydrochloric acid 20% H 3820% hydrochloric acid at 38C.

20% H 38*-20% hydrochloric acid at 38 C. also containing from 0.5 to1.0% FeCl 20% H 80-20% hydrochloric acid at 80 C.

The corrosion rates given in each instance, unless otherwise indicated,are the mean rates for five 48-hour test periods determined inaccordance with A.S.T.M. procedures set out in Specification No.A279-44T of the Society.

than the above conditions.

8 TABLE 1v Corrosion Rates, Mils Penetration per Year Alloy 40% 10% 30%30% 20% 20% 20% EN EN BS S H 38 H 38 H 80 180 14 2, 470 2,129 1, 520 609 2, 900 207 1 203 15 8 60 25 20 1 430 13 4 l 26 1 240 l, 603 l, 60L 547441 488 12 18 1o 4 43 Nil 414 Nil 35 15 4 20 N11 237 10 34 75 117 n Durichlor ll 23 27 125 1 Dissolved.

Increasing the chromium content at the low silicon level does notcorrect poor corrosion. See corrosion rates for alloy L, Table IV. Atthe preferred silicon level (1415%) but high chromium level, both themechanical properties as well as corrosion resistance are adverselyaffected. (See alloy M, Tables III and IV.) And at higher silicon levels(17%-19% high chromium content (20%), the mechanical properties arecompletely deteriorated. (See alloy N, Table III.)

The foregoing limits of silicon and chromium also apply where molybdenumis present in the alloys. Molybdenum is known to improve the resistanceof high silicon iron alloys in chloride ion environments, and this istrue of the chromium containing high silicon iron alloys of theinvention. Highest corrosion resistance commensurate with adequatemechanical strength is obtained at from 2.5% to 3.5% molybdenum, in analloy having from 14% to 15% silicon, 3% to 10% chromium. Althoughresistance to nitric acid decreases with more than 3.5% molybdenum,alloys containing as much as 5% molybdenum are still practical forspecific uses. But where bet ter mechanical properties are desirable oressential, a molybdenum content of from about 0.5% to 1.0% gives bestover-all results.

In alloys containing molybdenum it is sometimes desirable to add a smallamount of boron as a grain refiner to alleviate a tendency of the metalto spell when ground to provide a finished surface. Boron tends toreduce the eifectiveness of chromium and, when present, the chromiumlevel should be maintained near the high limit. The upper limit of boronshould not exceed about 1%, as corrosion resistance is depreciated to anuneconomic level.

Remarkable improvement in corrosion resistance under impressed currentconditions, i.e. in anode service, is obtained by the addition ofchromium to high silicon iron alloys which are frequently used in thistype of service. As shown by Table V, the improvement in consumptionrates and freedom from selective attack occurs only at chromium levelsin excess of 2%. The data below represents results obtained in syntheticsea water at 93 C. which is extremely severe in its eifect on allimpressed current anodes of previously known alloys.

TABLE V Anode Test Data-Synthetic Sea Water93 C.

at 2 Amps/Sq. Ft.

Analysis, Percent Consumption Alloy Rate, /lbs./ Type of Attack amp. yr.Cr M0 0 1.0 Nil 0. 95 14. 82 Badly pitted. 2. 0 Nil 0. 95 5. 75 Do.

3. 35 Nil 1.00 0. 94 Uniform.

6.10 Nil 0. 02 1. 00 Do.

1.0 3.0 0.95 12.90 Badly pitted. 2.0 3.0 0.95 0.95 Slight pitting. 3. 102. 72 0. 96 1. 34 Uniform.

The importance of chromium in these anode applications is furtherillustrated in Table VI by results obtained in synthetic sea water at 79C. which is much less severe For purposes of comparison,

data for anodes of Duriron and Durichlor are also given.

TABLE VI Anode Test DataSynthetic Sea Water79 C. at 2 Amps/Sq. Ft.

Analysis, Percent Consump- Alloy 1gion Rate,

s.am r. Si Cr Mo I ply Duriron 14. 50 Nil Nil 0.90 33 1. 40 Nil 0.9211.0 2.0 Nil 0.88 4.5 2. 30 Nil 0. 79 1.07

Nil 3.0 0. 90 1.0 2.88 0.98 26.4 1.38 2. 84 0.97 4.2 2.00 2.86 1.0 3.3

1 Tests of these alloys were discontinued before completion because ofeitlcessrvle rates of selective attack which rendered the anodessubstantia y use ess.

The criticality of the minimum 2% chromium level is well illustrated bythe foregoing. Selective attack is so excessive as to causesubstantially complete destruction of the anodes where no chromium ispresent, and that type of attack is largely eliminated only when the 2%level is reached. It will be appreciated that where selective attackoccurs, this can render an anode useless, due to breaking off of lengthsor large chunks, even though the normal consumption rate may not beexcessive. A maximum consumption of around two pounds per ampereyearrepresents an economic limit, and generally a rate of less than onepound per ampere-year is desired.

For special applications, as where the alloy is exposed to some of themost destructive types of corrosive media, for example boilinghydrochloric acid, inclusion of columbium along with molybdenum isuseful. For the most part, additions of elements other than chromium andmolybdenum to high silicon cast iron, provided these are not over %,havelittle effect upon the corrosion resistance in the more usual corrosivemedia. But as corrosive conditions become more severe, chemical attackon the alloys increases with the addition of such elements as, and inthe approximate amounts of 3% nickel, 5% cobalt, 5% copper, 3% antimony,2% titanium, 8% manganese and 3% aluminum.

As is evident from the foregoing the invention alloys are sharplydistinguished from the more conventional stainless or stain resistantalloys, as many are often referred to in the art. The character ofcorrosion resistance possessed by the new alloys here disclosed is of anentirely different order from that needed or contemplated for theso-called stain resistant low-silicon ferrous alloys. In the context ofcorrosion resistance here contemplated, a corrosion rate of 20 milspenetration per year is acceptable for use in most chemical equipmentfabrication although in some instances involving very severe conditionsa 50-60 mil per year rate will suffice. But in some exceptionallycritical areas a 5 mil per year rate maximum may be necessary. As agroup, the alloys of this invention are unique in meeting therequirements in respect to corrosion resistance and at the same timemaintaining adequate strength, thermal shock resistance and othermechanical properties to make them practically useful.

The alloys of the invention may be heat treated in the usual manner forhigh silicon cast irons, and do not require any change in establishedfoundry procedure or require any special care.

This application is a continuation-in-part of our prior copendingapplication Serial No. 836,885, filed August 31, 1959, now abandoned.

What is claimed is:

1. A corrosion resistant high silicon cast iron containing from 12% to17% silicon, 2% to 15% chromium and more than 0.1% to about 3.0% carbon,the balance being substantially all iron; said cast iron within therange of 10 alloy compositions represented by the area bbcc' in theaccompanying drawing having a carbon content always in excess of thevalue defined by the expression:

C=1.660.063 (percent Cr) 0.089 (percent Si) to provide free graphiterandomly dispersed in the ferrous matrix.

2. A corrosion resistant high silicon cast iron containing from 12% to17% silicon, 2% to 15% chromium, more than 0.1% to about 3.0% carbon, upto 5% molybdenum, the balance being substantially all iron; said carbonbeing present in amount within said range suflicient to provide freegraphite randomly dispersed in the solidified ferrous matrix.

3. A corrosion resistant high silicon cast iron containing from about14% to 15% silicon, from about 3% to 10% chromium and carbon from about0.2% to 1.5%, the balance being substantially all iron; said cast ironwithin the range of alloy compositions represented by the area b-bcc' inthe accompanying drawing having a carbon content which is always greaterthan the value defined by the expression:

C: 1.660.063 (percent Cr) -0.089 (percent Si) to provide free graphiterandomly dispersed in the ferrous matrix.

4. A corrosion resistant high silicon cast iron as defined in claim 3,which also includes up to 1% molybdenum.

5. A corrosion resistant high silicon cast iron containing from 14% to15% silicon, from 3% to 10% chromium (and from 0.8% to 1.5% carbon, thebalance being substantially all iron, said carbon being present in partin said alloy as free graphite randomly dispersed in the ferrous matrix.

6. A corrosion resistant high silicon cast iron containing from 14% to15 silicon, from 3% to 10% chromium, up to 3.5% molybdenum and from 0.8%to 1.5% carbon, the balance being substantially all iron, said carbonbeing present in amount within said range sufiicient to provide freegraphite randomly dispersed in the ferrous matrix.

7. A corrosion resistant high silicon cast iron containing about 14.5%silicon, 4.5% chromium, 0.5% molybdenum, 1% carbon, the balance beingsubstantially all iron.

8. An impressed current anode formed of high silicon cast iron alloyhaving a compositional analysis of 12% to 17% silicon, 2% to 15%chromium and 0.1% to 3% carbon, the balance being substantially alliron; said alloy within the range of compositions represented by thearea b'b-c--c' in the accompanying drawing having a carbon content whichis always greater than the value defined by the expression:

C=1.66-0.063 (percent Cr) -0.089 (percent Si) to provide free graphiterandomly dispersed in the ferrous matrix, said alloy having a transverseload capacity of at least 800 pounds.

9. An impressed current anode as defined in claim 8, wherein thecomposition of said alloy is from 14% to 15% silicon, 3% to 10%chromium, 0.2% to 1.5% carbon, the balance being substantially all iron.

10. An impressed current anode as defined in claim 8, wherein said alloyalso includes up to 1% molybdenum.

11. An impressed current anode formed of high silicon cast iron alloyhaving a compositional analysis of about 14.5% silicon, 4.5% chromium,0.5% molybdenum and 1.0% carbon, the balance being substantially alliron, said alloy having a transverse load capacity of at least 800 psiReferences Cited in the file of this patent Tetsu to Hogane (inJapanese), 1957, vol. 47, June, pages 652 to 657.

The Journal of the Iron and Steel Institute (British), No. 2, 1956, vol.183, page 338.

1. A CORROSION RESISTANT HIGH SILICON CAST IRON CONTAINING FROM 12% TO17% SILICON, 2% TO 15% CHROMIUM AND MORE THAN 0.1% TO ABOUT 3.0% CARBON,THE BALANCE BEING SUBSTANTIALLY ALL IRON; SAID CAST IRON WITHIN THERANGE OF ALLOY COMPOSITIONS REPRESENTED BY THE AREA B''-B-C-C'' IN THEACCOMPANYING DRAWING HAVING A CARBON CONTENT ALWAYS IN EXCESS OF THEVALUE DEFINED BY THE EXPRESSION: